Spin waves in bits observed for the first time
Every time you write a bit on a hard disk, countless waves run through the bit for several picoseconds; these are called spin waves. It is almost impossible to determine exactly how these waves influence the flipping process of the entire bit. Researchers at the Kavli Institute of Nanoscience of TU Delft have successfully observed spin waves for the first time in a bit that they created themselves with just six atoms. This study, which is published in Nature Materials on 6 July 2014, was partly funded by the FOM Foundation.
When a magnetic material is at rest, the magnetic fields of all atoms in the material are organised neatly – all north poles in the same direction, for example. If, however, the magnet loses its equilibrium, even only slightly, these magnetic fields (spins) will be thrown off balance, resulting in various wave movements. Insight into these spin waves is essential for determining how magnetic materials work and how we could improve them. This is not easy, however – spin waves occur at the length scales of only a few atoms and on the time scales of picoseconds.
Atom by atom
If you make the bit itself small enough, the length scale should not be a problem. With the help of a scanning tunnelling microscope (STM), in which a sharp metal needle scans atoms, Delft scientists can arrange iron atoms into a form of their choice, one by one. "By varying the distance between the atoms, we can accurately coordinate their magnetic interactions", says project leader dr. Sander Otte. "In this way, we design a bit with a composition that is clearer to us than the composition of any other bit. It requires some patience, but when the six atoms are finally in place, we are able to study them at our leisure, sometimes for weeks." The latter is possible due to the fact that the experiments will be conducted at a temperature below 1 Kelvin.
Flipping
The time scale is a more difficult problem. "We can easily detect the flipping of the bit, but it occurs so quickly that even the fastest electronic equipment makes it look like an instantaneous process. In order to observe the waves, we must make use of circumstantial evidence. By injecting an electrical current into different atoms in the bit, we can determine where and at which energy the largest increase in the flipping frequency occurs. The result is a beautiful wave pattern", explains Otte.
Quantum magnet
This research is not just technologically important. By understanding magnetic interactions at an atomic scale, Otte hopes to construct minuscule magnets in which the spins are not well organised at rest, as a result of quantum uncertainty. This type of quantum magnet still mainly exists on paper.
Video
On YouTube you will find an impression showing the manner in which a spin wave influences the flipping of an atomic bit. If the bit gets into an excited state at the node of the wave (in the middle of the atomic chain), not much will happen. The entire bit will only flip if the wave gets excited at an antinode (in this case, at the edge of the chain).
Reference
Nature Materials, Advance Online Publication (AOP) 6 juli 2014: 'Imaging of spin waves in atomically designed nanomagnets', A. Spinelli, B. Bryant, F. Delgado, J. Fernández-Rossier and A.F. Otte.
Contact
Dr. Sander Otte, Kavli Institute of Nanoscience, TU Delft, +31 (0)15 278 89 98
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